This invention relates to an improvement in self-sealing tubeless pneumatic tires constructed upon a laminate of elastomers comprising an air-impervious liner or elastomeric strip ("inner liner") coated with a thin layer of bare (that is, not covered and not constrained by a sheet of flexible material), elastomeric puncture-sealing material ("sealant"). The laminate of elastomers, namely the inner liner coated with the sealant is referred to herein as the elastomer laminate ("laminate" for brevity) in which the upper surface of the sealant is detackified with a detackifier which prevents adhesion of the surface to metal, and also to a hot, pressurized curing bladder. The uncured (uncrosslinked) sealant is essentially free of crosslinks. The laminate can be used as the first structural component of a tire to be constructed on a building drum because the detackified upper surface of the sealant does not stick to the drum.
In those instances where sealant is desired without the inner liner, this invention relates to a relatively thin generally laminar extrudate, less than about 10 mm thick, and at least 10 times as wide as it is thick, of an extruded sealant product which has unique flow (viscosity) properties. However, as extruded, the sealant strip has excellent adhesive qualities which must be countered by coating its upper and lower surfaces with detackifier. Thus surface-detackified, a desired length of the sealant product strip may be cut and placed on a building drum to initiate the building of a tire carcass. After the sealant product strip is placed upon the drum, the detackifier is removed from the upper surface of the strip, and then, an inner liner is conventionally wrapped over the strip, followed thereafter by successive layers which form the carcass.
This invention derives from the desire to eliminate the flow-restraining means referred to as "edge strips" in our copending patent application Ser. No. 044,628 filed May 1, 1987, and U.S. Pat. No. 4,664,168. The edge strips were necessary because we did not know how to formulate the sealant so that it would not flow under heat and pressure in the curing press. We were unaware of the critical importance of maintaining the weight ratio of high molecular weight (mol wt) elastomer to low mol wt elastomer greater than 1, nor were we aware of the role of the "tackifier" (so termed in U.S. Pat. Nos. 3,981,342; 4,064,922; and 4,115,172), having regarded the tackifier simply as a `processing aid` with little, if any, effect on viscosity.
Particularly, we used a mixture of tackifiers, including the same mixture of Struktol.RTM. 30 and Piccopale.RTM. resin as used in the '922 patent, to provide the particular balance of uncured and cured physical properties taught therein, recognizing that neither tackifier contributed to the cured Mooney viscosity of the sealant, but each greatly affected the properties of the sealant. Attempts to raise the viscosity of the uncured '922 sealant so that it would essentially not flow under curing conditions, yet to lower its cured viscosity so that it would seal punctures more effectively, prompted an investigation into numerous modifiers and processing aids. During this investigation, we found that, when we increased the ratio of high mol wt elastomer to low mol wt elastomer in the mixture, the viscosity increased, both before and after curing the mixture, which is what we expected. This increase in viscosity, obtained by increasing the relative amount of high mol wt elastomer, was too high to give satisfactory self-sealing of punctures. It was therefore particularly surprising that, as long as there was more high mol wt than low mol wt elastomer, by using more processing aid in the sealant recipe than used in the '922 patent, whether the processing aid was a tackifier or homogenizer, the viscosity of the uncured sealant was increased, and that of the cured sealant was decreased so as to give excellent self-sealing.
Further, though acceptable results are obtained with several different tackifiers, and with several combinations of different tackifiers, the excellent results obtained with a homogenizer in combination with a tackifier was overlooked. In particular, the homogenizer's peculiar role relative to the amount of tackifier, in influencing viscosity under heat and pressure was not appreciated.
A tackifier is a material which increases tack but has little effect on end properties after cure. A homogenizer is a material which improves the blending characteristics of elastomers of dissimilar polarity or viscosity, also with little effect on end properties after cure. Because tackifiers and homogenizers are low mol wt compounds it was to be expected that they would have no effect on the cured properties of the sealant mixture (recipe). Consistent with their combined presumed `non-effect` on cured properties, the teaching of the '922 patent regarded its combination "tackifier" as being a diluent for the low mol wt elastomer, requiring that the sum of the weights of low mol wt elastomer and tackifier be greater than that of the high mol wt elastomer.
It is therefore particularly surprising that, as long as there is a major proportion of high mol wt elastomer present, relative to the low mol wt material, it is not critical which tackifier or homogenizer, or what combination of tackifiers, or of homogenizers, or which combination of homogenizer and tackifier is used, provided the peak Mooney viscosity of the recipe is maintained in the range specified. This peak Mooney viscosity at 75.degree. F. (room temperature) ("MLP/rt") is in the range from above 70 to about 110, and is best maintained with a weight ratio of homogenizer to tackifier in a specified range. In this range, the pronounced effect on viscosity, both before and after curing the sealant mixture, is such that the edge strips of our '168 patent can be eliminated.
As was disclosed in the '922 patent to Farber et al., a mixture of high and low mol weight elastomers, the latter being present in an amount more than 50% by weight based on the combined weight of the high and low mol wt elastomers, and tackifier or plasticizer, and, cured to a limited extent, could adequately control flow under conditions of use of (running) the tire. The high mol wt elastomer furnished rigidity and strength, and the low mol wt furnished adhesion and conformability. By increasing the proportion of high mol wt elastomer, the tendency of flow attributable to the low mol wt elastomer is decreased but not completely removed. Therefore, the '922 mixture was partially cured, allowing the high mol wt elastomer to act as supporting structure to retard flow, without crosslinking the low mol wt elastomer to the point where its ability to function as sealant would be significantly impaired.
The function of the low mol wt elastomer as sealant was misdirected. We have found that, as is illustrated in an extreme case to establish the fact, there may be only one-tenth as much low mol wt elastomer as high, provided there is an appropriate choice of tackifier(s); in the best mode, there is more homogenizer than tackifier present.
Moreover, the flow properties of uncured sealant as now formulated, permits it to be cured in a green carcass with minimal flow onto the sidewalls of the carcass as it is vulcanized "cures") in contact with a hot pressurized bladder in a curing press, only because the detackifying function of the detackifier is not vitiated by the heat and pressure. Because the uncured '922 sealant, as an integral part of a green carcass, could not be cured in a curing press, the less preferred embodiment of the '922 invention required that a flat strip of extruded sealant be first cured, then incorporated on top of the liner in an uncured steel belted radial tire which was cured in a conventional tire press. In example VII of the '922 patent, the strip was cured by irradiation with a 1.4 million volt electron beam at a dosage of 20 megarads. The cured strip was placed on top of the liner inside a green tire, and the tire could be cured in a conventional tire press because the bladder did not adhere to the cured sealant on the liner. But curing the sealant on the inner liner also cures the inner liner with predictable results in a tire using the cured laminate.
It became imperative that the laminate not be precured for several reasons. To begin with, precuring the laminate resulted in using a precured inner liner to build a tire. One skilled in the art will appreciate that when a tire is expanded in the second stage, a precured inner liner will not have the elasticity required of it. But no attempt was made to cure an uncured laminate partly because it was evident that the sealant flowed uncontrollably during cure, and partly because no detackifier was found which readily provided the several features demanded of it. A detackifier was required to prevent adhesion of the sealant surface to a metal building drum, and also to prevent adhesion of the curing sealant (at about 350.degree. F.) to the hot pressurized curing bladder under typical operations in a conventional curing cycle. It is evident that if the sealant adhered to the drum, whether it adhered to the curing bladder was immaterial. In addition, the detackifier was to remain on the surface of the cured sealant, rather than being transferred to the curing bladder, because a non-detackified sealant surface is deemed more desirable to seal a puncture caused by a nail which, after piercing the tire's tread and sealant, is ejected from tire.
Hence, in the more preferred embodiment of the '922 invention, sealant (referred to as "post-cure sealant" because sealant was deposited in a cured tire) was extruded onto the inner liner of a cured tire, as described in detail in U.S. Pat. No. 4,115,172 to Baboff et al., so that the sealant adheres to the inner circumferential surface of the crown, on the opposite side of the road-contacting tread surface, and the sealant was cured by heating for several days at a temperature high enough to cure the sealant, typically about 150.degree. F., because the curing agent (curative) used was a tetraalkyl titanate.
Later attempts to cure an uncured laminate were made after the discovery of a commercially available material was found to be an effective detackifier. Still, all attempts to cure the laminate in the curing press resulted in unacceptable, if not uncontrolled flow of the curing sealant in the tire being cured. We addressed the flow problem during cure in our '168 patent by the use of flowrestraining edge strips.
The equivocal results obtained, coupled with the adverse economics of such post-cure sealant deposition, particularly as taught in the '342 and '922 patents to Farber et al., was followed by depositing the sealant before molding and curing a green tire carcass, referred to as pre-cure sealant; but the sealant was crosslinked to increase its viscosity, then was covered with a layer of flexible material to prevent adhesion of the sealant to the drum, as disclosed in the '342 and '922 patents. Covering the sealant caused blistering during cure. It became necessary in the prior art patents, to adhere the sealant and flexible material to the tire in a separate operation, after the tire was molded and cured.
Eventually, the economics of the additional operations for a crosslinked sealant being deposited after curing, led to a partially uncovered sealant constrained by edge strips. The edge strips restrain flow of the sealant during contact with a curing bladder under curing conditions, namely about 350.degree. F. and about 350 psig. This latter concept, utilizing the '922 sealant composition, is disclosed in our '168 patent.
In the best mode, we have added the homogenizer to the ingredients of the '922 sealant because we found that a recipe in which the sum of the weights of homogenizer and tackifier is greater than that of any other single component of the recipe, increases the viscosity of the sealant so as to essentially negate its flow during curing of the tire, yet permits the cured sealant (after the tire is cured) to flow enough to seal a puncture during use of the tire mounted on an operating vehicle.
The sealant laminated to an inner liner, disclosed in the '922 patent, comprised a blend of a major proportion of a low mol wt liquid elastomer mixed with a tackifier or plasticizer, and a minor amount by weight of a high mol wt elastomer. The blend, when crosslinked, had a peak Mooney viscosity of from 30 to 55 ML at 150.degree. F. (MLP/150). By trial and error, Farber et al had determined that a crosslinked sealant composition with a peak Mooney viscosity outside the range specified, would be unusable as a sealant. If the viscosity was lower than 30 MLP/150, the sealant would tend to flow down from the shoulder and sidewall areas of the tire when it is run at high speed as well as out of the hole when the tire is punctured. If the viscosity was higher, it would not flow sufficiently to seal a puncture.
Even at the high end of the range, the sealant was too fluid under conventional curing-press conditions. We coped with the problem, as disclosed in our '168 patent by providing sealant-retaining edge strips, carried by the inner surfaces of the sidewalls of the tire. These strips functioned as flow-restraining means because the viscosity of the '922 sealant was low enough to flow during curing of the tire, and required to be restrained.
Notwithstanding the effectiveness of the edge strips the difficulty of reliably and reproducibly manufacturing tires with such edge strips resulted in unfavorable economics of production. The decision was made to eliminate the edge strips in favor of renewing the search for a crosslinked sealant composition which does not flow within the green carcass when it is being cured in a curing press, and the bare surface of which crosslinked composition, upon curing, does not stick to the tire-building drum, yet after the tire is cured, the sealant is fluid enough to plug a puncture.
This invention embodies the culmination of that search.
The '342 patent, like the '922 patent, used a major amount of low mol wt elastomer relative to the high mol wt, but unlike the '922 patent, used no tackifier or plasticizer. The result obtained in the '342 patent was "to give an initial Mooney viscosity at room temperature (the initial peak reading attained which is usually within the first few seconds) of between 30 and 70 (large rotor, ML) in the final crosslinked mixture, with a preferred range of 40 to 60. Below 30 ML, the composition would tend to flow down from the shoulder and sidewall areas of the tire when it is run at high speed, as well as out of the hole when the tire was punctured. Above 70ML, the sealant capability of the composition is sufficiently impaired to render it unuseable for practical purposes." (see btm of col 3). The viscosities given were incorrectly given as being at room temperature when in fact they were measured at 150.degree. F.--a correction made in the '922 patent discussed hereinbefore.